When a user equipment (UE) is in idle state, its location is only known by the network at routing/location/tracking area level (rather than at cell level when the UE is in active state).
In LTE, for example, the tracking area code (TAC) is a configurable network identifier that is composed of at least one cell, but which can also comprise a number or cells or base stations/transceiver nodes (eNBs). This includes multi-frequency eNBs, which are considered to have one cell per frequency band. Typically, an operator would configure a TAC to include a large number of cells to cover a region or a city. As the UE only signals to the network when it updates its TA (by means of a tracking area update, TAU), this ensures the signalling load is kept low in areas where the number of UEs is large.
In the idle state, the network only has knowledge of the location of a UE at TA level. If the core network needs to send a paging message to this UE, it will have to instruct all cells included in the TA(s) in which this UE is registered to do so. This can result in a large paging overhead.